Heat-resistant high-strength stainless steel



Oct. 27, 1964 A. KASAK ETAL 3,154,412

HEAT-RESISTANT HIGHSTRENGTH STAINLESS STEEL Filed Oct. 5. 1961 4 Sheets-Sheet 1 TENSILE STRENGTH AT ROOM TEMPERATURE 300- o 25% FREE FERRITE AT2000F o YIELD STRENGTH AT ROOM TEMPERATURE T -13 o ,A U (ENSILE STRENGTH AT "00F 200- a Q. o a I 8 1 YIELD STRENGTH AT "00F 5 6O /A x A I00 20 X/+ 0 F5 F e 50E X U) 50- fi Fi l 40 g TEMPERING AT I200F c E E o I I l l 30 I o 2 4 e e PERCENT MOLYBDENUM I|OOF 75000 psi 'a 5 I Fig.2 [LI E 200- LLI II D 5000 i D A\ \LIZOO F 3 ps 0:

o l l i T 0 I 6 a INVENTORS PERCENT MOLYBDENUM AugustKusok Edward J. Dulis BY Vijuy Chondhok 1964 A. KASAK ETAL HEAT-RESISTANT HIGH-STRENGTH STAINLESS STEEL 4 Sheets-Sheet 3 Filed Oct. 5. 1961 Fig.5

mu 0 w w m 3 2 w. NED-FED PERCENT VANADIUM HOT WORK STEELS STEEL 7 7 l2l Cn HIGH STRENGTH STEELS SEMIAUSTENITIC STEELS m w w @8025 :GzmEm umzizmiwwmo 561-09 TEST TEMPERATURE (F) Fig.6

TILT A. KASAK ETAL HEAT-RESISTANT HIGH-STRENGTH STAINLESS STEEL Oct. 27, 1964 4 Sheets-Sheet 4 Filed 00L 5. 1961 Fig.7

HOT WORK STEEL SEM IAUSTENITIC STEELS 7 a 2 3 coo; :BzmEm 352M;

TEMPERATURE (F) 7. 7 L E E T S 2 4 E L w 0 C 5 m C M k 02 2 Z30 FIOG? Fig.8

TIME (Hours) United States Patent 3,154,412 EEAT-RESESTANT HIGH-STRENGTH STAINLESS STEEL August Kasak, Bridgeville, and Edward J. Dulis and Vijay K. Chandhok, Pittsburgh, Pa, assignors to Crucible Steel Company of America, Pittsburgh, Pin,

a corporation of New Jersey Filed Get. 5, 1961, Ser. No. 143,157 9 Claims. (Cl. 75-126) This invention pertains to stainless steel and particularly to stainless steel exhibiting very high strength at room and elevated temperatures. More particularly, the invention pertains to stainless steel having very high strength and exhibiting good oxidation and corrosion resistance under substantially atmospheric conditions at both ambient and elevated temperatures.

Concerning the various steels of the prior art, those known as austenitic stainless steels may be characterized as resistant to corrosion and oxidation and as having good strength at elevated tempertaures. A disadvantage of such steels, however, is their relatively low strength at ambient and moderately elevated temperatures. On the other hand, some steels of a heat-treatable nature and having a martensitic structure attain desired high strengths at ambient temperature which are retained at elevated temperatures. Exemplary of such steels are those covered by US. Patent No. 2,986,463, issued May 30, 1961, which was copending herewith. These steels, however, are not characterized by good resistance to corrosion and oxidation. Other martensitic steels having chromium contents of about 12% by weight have improved corrosion and oxidation resistance While similarly exhibiting relatively good strength at low temperatures and fairly high strength at elevated temperatures. The latter martensitic steels, however, are limited in chro mium content to a maximum value of about 12%. Above this value, the microstructure of the steel is characterized by the presence of substantial amounts of free ferrite. Free ferrite in substantial amounts is undesirable, being generally regarded as the probable cause of many hot-working diificulties as well as excessive variations in product properties with regard to the direction of working.

Another group of heat-treatable stainless steels, the so-called semiaustenitic steels, have chromium contents up to about 17%, but their applicability at temperatures above about 900 to 1000 F. is very limited. At such temperatures, particularly after prolonged exposure, these steels have relatively low strength values.

In accordance with a principal object of the present invention, steels are provided which are heat-treatable to very high strength levels and which retain an unusually high percentage of such strength at temperatures up to 1200 F. or higher. These steels offer particularly high strengths, in both short-time and prolonged exposures, at temperatures ranging from about 1000 to 1200 F. and

in addition exhibit high resistance to corrosion and oxidation. The inventive steels constitute a substantial improvement over the elevated-temperature structural materials known to the art and are particularly well suited for construction of aircraft and engine parts subjected to wide variations in operating temperatures.

The steels of the invention comprise iron-base alloys of the following broad composition ranges:

Percent Carbon 0.01 to 0.25 Nitrogen 0.01 to 0.25 Sum of carbon and nitrogen 0.06 to 0.35 Manganese Up to 1.0 Silicon Up to 1.0 Chromium 11 to 16 Vanadium Up to 1.0 Molybdenum Up to 10 Tungsten Up to 10 Sum of molybdenum and tungsten 3.5 to 20 Cobalt 10 to 20 Aluminum Up to 0.25 Boron Up to 0.025

Preferred composition ranges of the inventive steels are asfollows:

0.05 .OS'to 0.17% 0. 01 to 0.15%. 0.10 to 0.30%. 0.10 to 0.40%. 0.10 to 0.40%. 12.0 0 to 16.0%. Up p to 1.0%. 4.0 t to 6.0% Up p to 5.0% 12.0 .0 to 12.0%

In addition to the elements listed in the above broad and preferred composition ranges, the steels may contain impurities commonly encountered in the st%lmaking practice (such as sulfur and phosphorus) and tr-amp elements.

The steels of this invention are typified by Steels 77 and 107, the chemical compositions of which, together with those of a sizcable number of steels of similar compositions are given in Tables LA and LB. Table I-A lists these steels according to significant compositionalvariable, whereas Table I-B lists the same steels according to identification number. Microstructural characteristics of the steels as determined by microstructur'al examination and mechanical properties of the steels as determined by hardness, tension, and creep-rupture tests are set forth in Tables II through VI.

9 4.9 TABLE I-A Clzemzcal Composzttons Steels Investzgated 1 Steel 0 Cr V Mo Co N Al Other 0.177 13.44 0.46 3.01 19.46 0.04 0.042 2.36 W. 0.162 14.50 0.52 0.04 13.21 0.04 5.11 W. 0.154 14.26 0.41 5.09 13.64 0.033 0 025 0.007 B. 0.15 14.56 0.48 5.34 0.02 0.023 3.32 N7. 0144 13.80 0. 43 5.25 0.02 0.022 7.52 Nl. 0.167 14.46 0.46 5.09 0.04 0.037 0.084 10.14 N7. 0.164 14.46 0.41 5.12 13. 41 0.026 0.046 0.3611411. 0.161 14.50 0. 41 5.09 13.41 0.024 0.037 0.67 S1. 0.10 14.64 0.43 5.12 13.41 0.037 0.037 3.5;

1 Grouped according to significant compositional variable;

the significant compositional variable or variables are italicized.

TABLE I-B Chemtcal Composztzons 0f Steels Investzgated 1 Steel 0 Cr V Mo C0 N A1 Other 0. 15 14. 02 0.38 4. 83 18. 70 0. 02 0.177 13.44 0. 46 3. 01 19.46 0. 04 2.85 W. 6. 155 14. 26 0. 46 4. 93 10. 55 0. 03 0. 16 14. $6 0. 48 4. .90 18. 60 0.05 0. 155 13. 56 0.40 4. 7O 16. 20 0. 04 0.17 14. 48 0. 41 5. 22 11. 41 0.12 0. 08 14. 48 O. 46 4. 99 13. 56 0. 071 0. 12 14.40 0. 41 4.99 13. 45 0.028 0. 21 14. 52 0. 43 5. 09 13. 48 0. 025 0. 21 14.58 0. 46 5.02 13. 45 0.018 0. 15 14. 60 0. 41 5. 02 13. 48 0. 022 0.07 14. 48 0. 46 5.06 13. 49 0. 15 0. 145 15. 54 0. 41 4. 99 13. 41 0. 03 O. 16 16. 20 0. 41 5. 09 13.41 0. 03 0. 16 14. 36 0. 0 5. 15 13. 45 0. 028 0. 16 14.34 0. 87 5.09 13.45 0. 038 0. 157 14. 36 0. .97 3. 01 13.33 0.025 0. 146 13. 72 0. 41 0. 15 13. 11 0. 025 0. 16 14. 36 0. 41 3. 04 13. 64 0. 026 0.166 14.24 0. 41 7.15 13. 60 0. 026 0.154 14.26 0. 41 5.09 13.64 0.033 0.007 B 0. 154 14.36 0. 41 5.09 12. 20 0.03 0. 164 14. 40 0. 41 5. 28 11. 74 0.18 0. 164 14. 46 0. 41 5. 12 13. 41 0. 026 0.36 M77. 0.161 14.50 0. 41 5.09 13. 41 0.024 0.67 Si. 0. 16 14.64 0. 43 6.12 13.41 0.037 0.61 Mn, 0.79 Si. 13.98 0. 46 5.09 0. 02 0.057 14. 04 0. 43 4. 96 3.98 0.032 14. 04 0. 43 4. 99 8. 08 0. 03 14. 46 0. 46 5.09 0. 04 0.037 10.14 N2 14. 48 0.48 4. 96 13.56 0. 016' 14.48 0. 46 4. 99 13.56 0.016 14. 56 0. 48 5. 34 0. 02 0. 023 3.82 Ni. 13.80 0. 43 5. 0. 02 0. 022 7.52 Ni. 14. 16 01 5. 38 13. 43 0.054 14.50 0. 52 0. 04 13. 21 0. 04 5.11 W.

1 Steels llsted in numerical order.

TABLE II Results of Mlcrostructural Examtnation and Hardness Tests 1 Austenitized at 1,900 F. Austenitized at 2,000 F.

Hardness (Rn) after Hardness (RD) after Steel Free Tempering 2+2 Free Tampering 2+2 Ferrite hr. at: Ferrite hr. at: (percent) (percent) 1,100 F. 1,200 F. 1,100 F. 1,200 F.

0 48 48 0 48 48 0 52 49 0 49 50 5 51 44 10 52 46 0 53 47 0 54 47 O 54 49 O 55 49 0 54 50 1-5 56 5.5 1 54 48 0-1 54 51 1 53 47 0-1 53 49 0 53 47 0 55 50 0 53 47 0 56 51 0 53 47 l 54 50 0 54 50 0 54 52 0 52 49 5 5") 49 10 52 48 41 41 0 53 48 O 54 50 1 53 47 10 55 51 O 47 0 47 41 0 36 33 0 36 34 0 47 39 0 46 42 0 55 49 25 56 53 0 53 47 0 55 1 51 46 5 53 48 0 54 49 0 56 51 0 49 49 1-5 49 0 49 49 1 57 52 0 51 52 0 41 50 100 18 17 24 23 50 35 33 50 36 37 30 44 43 30 49 43 0 R1,, 87 R 89 0 R1,, 84 R1,, 83 29 50 46 30 47 30 48 44 40 47 43 50 38 35 50 41 37 0 R 92 R 92 5 R 87 R 85 0 50 44 20 5 45 0 49 45 0 50 46 1 Heat treatment: Austenitized at the indicated temperature (1,900

40 or 2,000 F.), oil quenched, refrigerated at F. for 111:, and tempered at 1,100 F. for 2+2 hr.

TABLE III Results of T enszon Tests at Room Temperature 1 Austenitiz 0.2% Ofiset Tensile Elong. in Reduction Steel ing 'Iem- Yield Strength Inch 01 Area 50 perature Strength (1,000 p.s.i.) (Percent) (Percent) F.) (1,000 .s.i.)

Heet treatment: Austcnitized at the indicated temperature, oil guenlehed, refrigerated at 100 F. hr., and tempered at 1,l00 F.

7 room temperature, and heating at an elevated temperature :below about 1500 F. Refrigeration at a subzero temperature, e.g., 100 F., is preferably included in the heat-treating schedule immediately following the austenitizing step for the purpose of facilitating the transformation of austenite. While the recited sequence of heattreating steps has been found admirably suited for the contemplated purpose, it is well to note that other sequences of heat-treating steps, including conditioning treatments, may be employed, the principal criteria being the complete or substantially complete elimination of free ferrite at elevated temperatures and the complete or substantially complete elimination of austenite thereafter as well as the obtention of high strength.

It is also understood that a combination of thermal and mechanical treatments may result in further improvement of some of the important properties of the inventive steels. For example, when a sheet sample of Steel 77 was cold reduced 50% in thickness and subsequently heated at 1100 F. for two plus two hours, a tensile strength of 341,000 p.s.i. and an elongation of 6% were obtained at room temperature. In contrast, it will be noted from Table III that thermal treatment alone of Steel 77 resulted in a maximum tensile strength of only 294,000 p.s.i.

It is well known to the art that in order for a steel to exhibit satisfactory corrosion resistance at ambient temperature and satisfactory oxidation resistance at temperatures up to about 1200 F., the chromium content of such steel should be about 12% or more. In general, the higher the chromium content of the steel, the higher its corrosion and oxidation resistance. In view of these considerations, chromium has been included as an essential element in the steels of the invention, the minimum content thereof being set at 11%. As it is desirable to maintain the chromium content at the highest level compatible with desired microstructural characteristics, i.e., essential freedom from free ferrite and retained austenite, the maximum chromium content of the inventive steels has been set at about 16%. The latter figure finds support in the accompanying data wherein it is ob-' served from Tables I-A and H that when the compositional variable is chromium, values of 14.36% (Steel 77) and 15.54% (Steel 97) result in a microstructure having no free ferrite after an austenitizing treatment at 1900 F., whereas, a value of 16.20% (Steel 98) results in a microstructure having 10% free ferrite after the same austenitizing treatment.

Molybdenum has been included in the steels of the invention as an effective strengthener. Supporting data therefor were obtained by testing steels of the basic inventive composition wherein the molybdenum content was varied (see Table I-A, Steels 102, 103, 77 and 104). As shown in FIGS. 1 and 2, all the important strength properties, i.e., yield and tensile strengths at room temperature and at 1100 F., hardness after tempering at 1100" F. and 1200 F., and rupture times in creep-rupture tests at 1100 F. and 1200 F., are steadily enhanced as the molybdenum content is increased from about 0.1 to about 5%. Tables III through VI indicate that for this same molybdenum range, ductility is maintained at a high level.

resistance of the invention steels in such media as chloride solutions.

Tungsten is also an effective elevated-temperature strengthener in the inventive steels. For example, a steel containing 5.11% tungsten and only 0.04% molybdenum (Steel 122) exhibited a higher creep-rupture life (782 hours) than any other steel when tested at 1100 F. under a stress of 75,000 p.s.i. (see Table V). Similarly, when tested at 1200 F. under a stress of 35,000 p.s.i., this same steel again exhibited a higher creep-rupture life (175 hours) than any other steel tested (see Table VI).

In addition to the benefits derived from the individual uses of tungsten and molybdenum, test data show that a combination of these elements is an effective strengthening agent in the inventive steels. In support thereof, it will be noted that Steel 75, containing 3.01% molybdenum and 2.35% tungsten, exhibited a creep-rupture life of 365 hours when tested at 1100 F. under a stress of 75,000 p.s.i. (see Table V).

Cobalt has been incorporated as an essential ingredient in the inventive steels for the purpose of obtaining desired microstructural characteristics, a critical cobalt range of 10 to 20% having been determined therefor. In this regard, Table II strikingly reveals that cobalt values below about 10% (Steels 111, 112 and 113) result in the formation of excessive amounts of free ferrite, and that cobalt values above about 10% (Steels 76, 106, 77, 78 and 67) permit the complete or substantial elimination of free ferrite by proper compositional balance and/or heat-treatment selection. On the other hand, cobalt contents in excess of about 15% permit the retention of increasing amounts of austenite, the effect of which is to lower the room-temperature strength properties of the steels to some extent. This is graphically depicted in FIGS. 3 and 4, wherein it is observed that optimum strength values at both room temperature and 1100 F. are obtained in a cobalt range of about 13 to 15% and that said values fall off with increasing cobalt content until an extrapolated maximum of about 20% cobalt is reached, beyond which value strength properties become unsatisfactory.

In addition to the employment of a critical cobalt range, the invention contemplates as a preferred embodiment a proper balancing of the cobalt content within said range with the nitrogen content. This proper balancing, as evidenced by Steels 76, 106, 77, 78 and 67, involves the restriction of the cobalt content to values in excess of 12% where the steel is essentially nitrogen-free or low in nitrogen, i.e., less than about 0.1%. Where the nitrogen present amounts to about 0.10% or greater, the cobalt content may be lowered to the range 10 to 12%. It will be noted from Table II after an austenitizing treatment at 1900 F. that Steel 76, embraced within the broad concept of the invention, exhibits 5% free ferrite and that Steels 106, 77, 78 and 67, comprising preferred embodiments of the invention, exhibit not more than 1% free ferrite after the same austenitizing treatment.

As is known to the art, the alloying elements cobalt and nickel are interchangeable to some extent for the formation of an austenitic structure and for the elimination of free ferrite in steel. As increasing amounts of nickel up to 10.14% (Steels 118, and 114) were used in the steels of this invention as a replacement for cobalt, it became evident that while nickel effectively caused the amount of free ferrite to decrease, it also stabilized the austenitic structure of the steel so that when the free ferrite was fully eliminated (as in Steel 114 containing 10.14% nickel), the steel was not hardenable and thus incapable of exhibiting very high strengths at ambient and moderately elevated temperatures. Therefore, it is recognized that although to some extent nickel may be substituted for cobalt in these steels, no complete or substantial replacement of cobalt with nickel is possible.

The elements carbon and nitrogen both constitute strong austenite formers and are essential in the steels of this invention for the elimination or minimization of free ferrite in the microstructure. Quantitatively, it appears that about 0.06 to 0.35% carbon plus nitrogen (combined) is required to avoid substantially the formation of free ferrite upon heat treatment at high temperatures.

9 However, it is important to select quantities within the foregoing range so as to maintain a proper compositional balance vis-a-Vis ferrite foriners, lest the ferritizing effect of the latter become dominant. For example, wherethe sum ofcarbon plus nitrogen is less than about 0.1% the presence of vanadium (a strong ferrite former) cannot be tolerated above about 0.1%. In this regard it will be noted that Steel 121 (containing 0.068% carbon plus nitrogen and no intentionally added vanadium) was heattreatable to zero percent free ferrite, while Steel 115 and 116 (containing 0.076% carbon plus nitrogen and 0.039% carbon plus nitrogen, respectively as Well as about 10 the effects with tungsten are about twice as great as those of molybdenum because of the substantial difference in the atomic weights of these two elements. In the steels of this invention we consider tungsten as at least a partial replacement for molybdenum.

For the purpose of comparing relative strength and corrosion properites, an investigation was undertaken along these lines with respect to thesteels of the invention and prior-art steels of the semiaustenitic, the 12% chromium high-strength, and the chromium hot-work tool-steel types. The nominal compositions of representative examples of the latter steel types are set forth in Table VII.

TABLE VII Nominal Compositions of Comparison Steels Designation I C l Mn Si Cr Ni V Mo W Al N Fe Semiausteuitie Steels:

AM 350 1 0.10 1. 0. 40 16. AM 355 0.15 1.00 0. 40 15. 5 17-7PH 0. 09 17. 0 PH-7Mo 0. 09 1. 0 1. 0 14.0-16.0

max. max. max. 12% Cr High-Strength Steels- Crucible 422 0.22 0.65v 0.36 12.0 0.70 0.25 1.00 1. 00 Bal. oruoib1o422M 0.28 0.84 0.25 12.0 0.20 0.50 2.25 1.70 Bal. Lapelloy 4 0.30 1.0 0. 12. 0 0.30 0.25 2. 75 Bal. Hot-Work Steels -11 a 0. 38-0. 43 0. 20-0. 40 0.80-1.00 4.75-5.25 0.40-0.60 1.20-1.40 Bal. Crucible 21s 5 0.35 0.35 1.00 6, 00 0.45 1.40 Bal.

1 Grade of Allegheny-Ludlum Steel Co. 2 Grade of Armco Steel Co. 3 AISI Type. 4 Grade of General Electric-C0. 6 Grade of Crucible Steel Co. of America.

one-half percent each of vanadium) were heat-treatable Comparative data relating to creep-rupture strength to no less than 20% and 30% free ferrite, respectively. and elevated-temperature tensile strength for the steels in On the other hand, it has been found that high carbon Table VII, representative of the types recited therein, and plus nitrogen contents have an adverse effect on ductility a sample of Steel 77, representative of the steels of the at high strength levels and promote the retention of invention, are given in Tables VIII and IX, respectively. austenite in the microstructure. Therefore, to ensure TABLE VIII proper compositional balance, it is preferred in the steels I of this invention to keep the carbon plus nitrogen content Compar f p- 47 e Strengths between about 0.10% and 0.20% when the content of cobalt (as indicated heretofore, also a strong austenite IOU-Hour Creep-Rupture Strength (1,000 psi.) former) is about 12% or higher, and between about 15 steel 0.20% to 0.30% when the content of cobaltis below 800 F. 900 F. 1,00DF. 1,100F. 1,200 1 about 12%.

Vanadium in amounts up to about 1.0% may be added SemilR/{Sliglgtic Steels: to the steels of this invention to improve certain properfi ties. As shown in FIG. 5, additions of vanadium to the 50 12 gfl r steels of the invention increased rupture life at 1100 F., z 1g rengt as determined by creep-rupture tests, the highest rupture 501 ga life being obtained With a steel containing about 0.5% Vanadium. Hot- 4 111 1 1 Steel: Cru- It is generally agreed that tungsten has alloying effects 55 Lfi g'iQQgE-{ E in steels quite similar to those of molybdenum except 5189177 that the amounts (in weight percent) necessary to produce TABLE IX Comparison of Elevated-Temperature Tensile Strengths Steel Condition R.T. I 200 300 400 500 600 700 800 900 1, 000 1,100 1, 200

F. F. F. F. F. F. F. F. F. F.

Semiaustenitic Steels:

AM-355 SC'I 222 208 20s 20s 197 180 200 185 191 179 190 179 165 68 45 238 227 211 203 182 161 12% Chromium High-S ength Steels:

Crucible 422 243 230 216 109 57 Crucible 422M 101 74 Hot-Work Steel: H 280 260 243 230 185 100 Steal of the Invention: Steel 77. 290. 286 259 233 214 176 121 1 Subzero-cooled and tempered. 2 Roll-hardened at room temperature and aged 950 F. 3 Queuehed and tempered.

Graphical depiction of the foregoing data is shown in FIGS. 6 and 7, wherein the superiority of the steels of the invention over the comparison steels of the prior art is clearly evident.

To assess the corrosion resistance of the steels of this invention, a heat-treated sample of Steel 77 was subjected to the water-vapor-column corrosion test and to the CASS test along with heat-treated samples of a number of the aforementioned prior-art comparison steels. These tests have been used for many years as laboratory tests to indicate the relative corrosion resistance of stainless steels under essentialy atmopheric conditions.

In the water-vapor-column test, the samples are exposed to water vapor condensation for a period of 8 hours and then allowed to dry for a period of 4 hours. This procedure is repeated several times. Results of this test after six wet-and-dry cycles showed the H-11 specimen to be covered with heavy rust, and the Crucible 422 steel to contain local rust spots, whereas it showed Steel 77 of this invention to be only slightly stained, as were the samples of AM 350 and PH15-7M0.

In the so-called CASS test, the specimens are sprayed with an acidified aqueous solution of NaCl and CuCl for 16 hours at 120 F. The samples are then cleaned with a detergent and the surface appearance is rated by visual examination. In this test the 11-11 sample was severely attacked and Crucible 422 was seriously pitted; Steel 77 along with other stainless steels showed only slight pitting and all to about the same degree.

To determine the oxidation resistance, samples of Steel 77 were exposed to a static air atmosphere at 1200 F. and measurements of weight gain were made periodically; in this test, a higher rate of weight gain indicates a higher rate of deterioration (scaling) of the steel in air at the test temperature. The results of this test, given in FIG. 8, show clearly that Steel 77 of this invention exhibited a superior resistance to oxidation attack.

For the purpose of evaluating welding characteristics of steels of the invention, welding tests were conducted on sheet samples of Steel 77 using the semiautomatic inert-gas-shielded-tungsten-arc process. The tests involved tension testing of one-eighth inch thick specimens, both welded and unwelded. The average results of these tests are given in Table X.

These results indicate a weld efiiciency (i.e., ratio of tensile strengths of the welded specimens to those of the unwelded specimens) of 98%. Also, it is important to note that no high-temperature heat treatment is needed after the welding operation. This feature is extremely important in fabrication, since it virtually precludes any ditficulties resulting from scaling and warping of a formed and welded article.

While the invention has been described and disclosed in connection with specific embodiments thereof, it will be understood that the invention is not limited thereto but may be otherwise embodied or practiced within the scope of the appended claims.

We claim:

1. A stainless steel having enhanced strength and useful ductility over a temperature range from subzero to about 1200 F., consisting essentially, by weight percent, of:

Element: Percent Carbon+nitrogen 0.06 to 0.35. Chromium From 11 to 16. Molybdenum-i- /z tungsten From 3 to less than 7. Nickel Up to 3. Vanadium Up to 1. Nickel-i-cobalt 10 to 20. Manganese Up to 1. Silicon Up to 1. Aluminum Up to 0.25. Boron Up to 0.025. Iron Balance.

wherein vanadium is present on the low side of its range when the sum of carbon-i-nitrogen is on the high side of the combined range of those elements.

2. A stainless steel having enhanced strength and useful ductility over a temperature range from subzero to about 1200 F., consisting essentially, by weight percent, of:

Element: Percent Carbon-knitrogen 0.10 to 0.30. Chromium 12 to 16. Molybdenum-l-Vz tungsten More than 3 to 6. Nickel Up to 3. Nickel+cobalt 12 to 16. Vanadium Up to 0.5. Manganese Up to 1. Silicon Up to 1. Aluminum Up to 0.25. Boron Up to 0.025. Iron Balance.

where the sum of carbon+nitrogen is in the range of 0.10 to 0.20 when the sum of nickel-l-cobalt is at least 12% and is in the range of 0.20 to 0.30 when the sum of nickelf-l-cobalt is less than 12%.

3. A stainless steel having enhanced strength and useful ductility over a temperature range from subzero to alf out 1200 F., consisting essentially, by weight percent, 0

4. A heat-treated stainless steel consisting essentially of the composition of claim 1, said steel being characterized by the following minimum mechanical properties:

Hardness Rockwell C 48 Tensile strength (room temperature) p.s.i. 2S0,000 Tensile strength (1100 F.) p.s.i. 170,000

5. A heat-treated stainless steel consisting essentially of the composition of claim 2, said steel being characterized by the following minimum mechanical properties:

Hardness --Rockwell C 48 Tensile strength (room temperature) p.s.i. 250,000 Tensile strength (1100 F.) p.s.i. l70,000

13 6. A heat-treated stainless steel consisting essentially of the composition of claim 3, said steel being characterized by the following minimum mechanical properties:

Hardness "Rockwell C 48 Tensile strength (room temperature) p.s.i. 250,000 Tensile strength (1100 F.) p.s.i. 170,000

7. A heat-treated stainless steel as in claim 4, said steel being additionally characterized by the following minimum mechanical properties:

Hrs. Creep-rupture life (75,000 p.s.i. at 1100 F.) 100 Creep-rupture life (35,000 p.s.i. at 1200 F.) 50

8. A heat-treated stainless steel as in claim 5, said 14.: steel being additionally characterized by the following minimum mechanical properties:

Hrs. Creep-rupture life (75,000 p.s.i. at 1100 F.) 100 Creep-rupture life (35,000 p.s.i. at 1200 F.) 50

References Cited in the file of this patent UNITED STATES PATENTS 2,848,323 Harris et a1. Aug. 19, 1958 2,880,085 Kirkby et a1. Mar. 31, 1959 2,990,275 Binder et a1. June 27, 1961 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 5 ,l54,4l2 October 27 1964 August Kasak et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, TABLE I-A, under the heading "V", line 10,.

thereof, for "3.48" read 0.48 same column, same table, under the heading "Al", line 34 thereof, for "0.14", in italics, read 0.042 in italics; column 7, line 71, for

"invention" read inventive column 9, line 10, for

"Steel" read Steels column 11, line 12, for "essentialy atmopheric" read essentially atmospheric o Signed and sealed this 27th day of July 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A STAINLESS STEEL HAVING ENHANCED STRENGTH AND USEFUL DUCTILITY OVER A TEMPERATURE RANGE FROM SUBZERO TO ABOUT 1200*F., CONSISTING ESSENTIALLY, BY WEIGHT PERCENT, OF: 